This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 90124658 filed in TAIWAN on Oct. 5, 2001, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a process for producing carboxylic acids, and more particularly to a process for producing acetic acids by the carbonylation of methanol with carbon monoxide. The process is characterized by introducing one or more specific organic compounds containing nitrogen and oxygen in the presence of a catalytic system containing rhodium catalyst. The organic compounds are used as cocatalyst for the catalytic system so as to increase rhodium concentration contained in a reaction medium, thereby carrying out the reaction within wider operation limits and under relatively severe conditions.
2. Description of the Related Art
The preparation of acetic acids by rhodium-catalyzed carbonylation of methanol and carbon monoxide is a well known process, which has been operated upon a commercialized scale for several decades. The process comprises carbonylizing an alcohol having n carbon atoms with carbon monoxide into a carboxylic acid having (n+1) carbon atoms in liquid phase in the presence of a dissolved homogeneous rhodium catalyst and a promoter such as an organic iodide compound.
In such process, carbonylation is usually carried out at an elevated temperature (for example 185xcx9c190xc2x0 C.) and at a high pressure (for example 20xcx9c35 kg/cm2). Since the rhodium catalyst is present in a form of rhodium (I), such unstable rhodium (I) catalyst is readily oxidized into an inactive form of rhodium (III) to be precipitated off under the above condition. Therefore, it is necessary to add appropriate amount of water and hydroiodic acid for enhancing rate of carbonylation and maintaining stability of the catalyst system. For example, it is suggested by Monsanto Company (one of the earliest companies to develop such production process of acetic acid) that this process is operated by using 14% of water and 1.17 mole/l of iodide.
It has been found that precipitation of insoluble rhodium (III) species has the tendency to occur when partial pressure of carbon monoxide is at a low level and the water used is less than 14% by weight. Since this commercialized process results in some problems, many patents are directed to methods for overcome the precipitation problem in the past several decades.
U.S. Pat. No. 4,733,006 discloses the use of inorganic salt additives XOAc (X=Li+, Na+, K+), however, the effect of the inorganic salt additives on reaction rate is not described.
GB Patent No. 1,538,783 discloses a process for producing acid anhydride by using Group IV B, V B, VI B or VIII B transition metal as a catalyst and organic trivalent nitrogen or phosphorous compound as a catalyst stabilizer. The organic nitrogen compound represents amine compound with a substituent of hydroxy, epoxy, aromatic, nitrile groups, etc. It is said in this patent that such organic compounds are effective to stabilize catalyst and reduce corrosion.
U.S. Pat. No. 4,336,399 mentions trivalent nitrogen-containing organic amine compounds, which are readily to form quaternary ammonium compounds in reaction mediums so as to increase the concentration of free iodine. Thus, the amine compound indirectly makes the catalyst system more stable, but the amine compound itself has no efficacy of directly stabilizing catalysts.
Moreover, some patents allows for using heterocyclic compounds having simply nitrogen as catalyst stabilizers, for example N-methylimidazole described in EP 0 153 834 and bipyridin described in U.S. Pat. No. 4,433,165. Another example is U.S. Pat. No. 5,442,107, wherein the nitrogen-containing heterocyclic compounds selected from the following are use as catalyst stabilizers in carbonylation of methanol at a low water content:
(1) 2-ethyl-4-methyl imidazole,
(2) 4-methyl imidazole,
(3) 4-t-butyl-pyridine,
(4) 2-hydroxy-pyridine,
(5) 3-hydroxy-pyridine, and
(6) 4-hydroxy-pyridine.
The nitrogen-containing heterocyclic compounds mentioned in the above three patents, however, will react with iodide in reaction mediums to form quaternary ammonium iodide salts under severe conditions such as low content of water and high concentration of organic iodine. The quaternary ammonium iodide salts can form insoluble complexes with rhodium and thus the catalyst stabilization is lost. Further, the insoluble complexes may be precipitated from the reaction solution of the carbonylation of methanol at a low water content. In these prior arts, it has not been mentioned or implied that the pyridine derivatives having substituents other than OH group and alkyl group have effects on reducing precipitation of the rhodium catalyst in the carbonylation of methanol at a low water content.
U.S. Pat. No. 5,144,068 teaches the use of inorganic iodide LiI as a catalyst stabilizer in the carbonylation of methanol. This stabilizer improves the precipitation of the rhodium catalyst under the condition of a low water content, and an approximately identical reaction rate to that of the reaction system at high water content (e.g. 14% by weight) is achieved. According to this patent, a quaternary ammonium salt, i.e. N-methyl-picolinium iodide, is employed at a low water content to increase the reaction rate of carbonylation. However, it is found that N-methyl-picolinium iodide readily reacts with rhodium to forms an insoluble complex, which may be precipitated from the reaction solution.
U.S. Pat. Nos. 4,670,570 and 5,488,153 teach the use of phosphorus-containing compounds such as [Pxcx9cPxe2x95x90S](CO)Cl, [Pxcx9cPxe2x95x90O](CO)Cl and [Pxcx9cCOCH3](CO)Cl as a material for stabilizing the rhodium catalyst. However, there still exist the following disadvantages: (1) although phosphorus is a strong ligand, one coordinate bond needs to be dissociated for exposing rhodium (I) species during the reaction, and thus the oxidation addition of methyl iodide can be carried out; in addition, the strength of each coordinate bond of the phosphorus-containing compound is substantially identical, which means that the whole phosphorus-containing compound might be dissociated from rhodium atom in the reaction system so as to result in the precipitation of rhodium. (2) The dissociated free phosphorus may react with trace oxygen and methyl iodide in the system to form phosphorus oxide and phosphorus iodide, respectively, and thus the inherent strong coordinating ability of phosphorus is lost. In addition, it is also disadvantageous to have relatively high activation energy for performing the reaction and add excess amount of triphenylphosphine to keep its activity.
EP 0 055 618 discloses the addition of an organic catalyst stabilizer into the reaction solution for reducing the precipitated amount of the rhodium catalyst in the carbonylation of methanol due to a low water content. The stabilizer employed in this patent is selected form one or more organic compounds containing one or more nitrogen atoms, phosphorus atoms or COOH groups:
(1) N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-o-phenylene diamine and 2,3xe2x80x2-dipyridyl;
(2) HOOCxe2x80x94Y1xe2x80x94COOH and (HOOCxe2x80x94Y2)(HOOCxe2x80x94Y3)Nxe2x80x94Y1xe2x80x94N(Y4xe2x80x94COOH) (Y5xe2x80x94COOH), where Y1-5xe2x95x90(CH2)m; and
(3) (R1)(R2)Pxe2x80x94R3xe2x80x94P(R4)(R5), where R1-5 is alkyl.
Since the type and number of these functional groups are not as good as those used in the present invention, such catalysts have relatively poor stabilizing effect.
Although some organic or inorganic salt additives for reducing or avoiding the precipitation of rhodium under low partial pressure of carbon monoxide and at a low water content are provided in the above-described prior arts, there still exist some drawbacks required to be overcome. Therefore, it is necessary to provide a cocatalyst for efficiently stabilizing a rhodium catalyst under a severe reaction condition of carbonylation so as to reduce the precipitation of rhodium.
The present invention relates to a process for producing a carboxylic acid. The process comprises carbonylating an alcohol having n carbon atoms, an ester of the alcohol and the carboxylic acid or a dialkyl ether having n carbon atoms in each alkyl group with carbon monoxide in the presence of a catalytic system containing a rhodium catalyst so as to produce the carboxylic acid having (n+1) carbon atoms, the process is characterized by using a reaction medium for the carbonylation comprises: (1) a rhodium catalyst; (2) an organic halide corresponding to the alcohol; (3) an ester of the alcohol and the carboxylic acid; (4) the carboxylic acid; optionally (5) water, a haloid acid, an inorganic halogen salt or an acetate; and (6) a cocatalyst selected from one or more nitrogen- and oxygen-containing organic compounds represented by the following formula: 
where R1, R2 and R3 are identical or different, and each represents: 
where
R4 represents H, an aliphatic group having 1 to 6 carbon atoms, or an arylaliphatic or an aromatic group having 6 to 10 carbon atoms;
U represents H, aliphatic group having 1 to 6 carbon atoms or an arylaliphatic group or aromatic group having from 6 to 10 carbon atoms;
V and W are identical or different, and each represents a covalent bond, an aliphatic group having from 1 to 6 carbon atoms or an arylaliphatic group or aromatic group having 6 to 10 atoms; and
X, Y and Z are identical or different, and each represents H, metal ion or an aliphatic group having 1 to 6 carbon atoms,
with the proviso that at least one of R1, R2 and R3 is a group other than R4.